1. Field of the Invention
The present invention relates to an image capture apparatus and method, and more specifically to an image capture apparatus having a so-called “wobbling” function that detects a focus state of the photographing lens by reciprocally moving a focus position of a photographing lens relative to an image sensor at a predetermined amplitude by changing the distance in the optical axis direction between the photographing lens and the image sensor, and an image capture method.
2. Description of the Related Art
Conventionally, a wobbling method used in contrast AF for detecting a focus state based on the contrast of captured images is known as a method for detecting a focus state to provide automatic focus functions (AF system), as disclosed in Japanese Patent Laid-Open No. 2006-146062. Such an AF system that controls focusing by the wobbling method generally operates as follows.
First, an in-focus state is attained by contrast AF (hill-climbing detection method), then an image is captured at regular intervals with a focusing lens being reciprocally driven (wobbled) within a narrow amplitude range centered on an in-focus position, and the in-focus position is determined based on the contrast of captured image signals. Then, in a case where the in-focus position is changed, if the change is small, reciprocating motion is continued after changing the center of the amplitude of reciprocal drive to the changed in-focus position, and if the change is large, focusing is controlled by the hill-climbing detection method.
In contrast AF, the contrast of images captured at, at least, two focusing lens positions is compared so as to determine a direction to the in-focus position. Thus, at the time of wobbling, once in-focus state is achieved, the contrast of images obtained at different lens positions by minutely driving the focusing lens is compared in order to maintain the in-focus state.
However, there is a problem with the wobbling method disclosed in Japanese Patent Laid-Open No. 2006-146062 in that, in a case where the focal plane of the focusing lens greatly deviates from the photoreceiving surface of the image sensor during wobbling, defocusing can be noticed by the photographer.
For this reason, it is a common practice to set the amount of change of the focusing lens during wobbling such that the amount of defocusing occurring becomes smaller than an acceptable circle of confusion, thereby preventing defocusing of an image from being noticeable to the photographer. However, there is still a possibility that defocusing or a change in defocusing during wobbling is noticeable to the photographer under conditions as follows:
(1) A case where a photographing lens with no capability for high-precision minute drive is mounted on an interchangeable lens camera;
(2) A case where an acceptable circle of confusion is considerably reduced in size due to the miniaturization of the image sensor;
(3) A case of detecting the focus of a moving subject.